In recent years, a high-speed interface, such as a universal serial bus (USB) and an high-definition multimedia interface (HDMI), has been upgraded to work with a higher speed. This market trend invites a problem of how to deal with radiated noise. A common mode noise may cause the unintended noises, so that the market may demand a common mode noise filter working at the higher frequency in order to remove common mode noises.
The common mode noise filter includes two coils wound in the same direction. An electric current flowing through a coil generates a magnetic field, so that a self-inductance produces a braking effect.
The two coils of the common mode noise filter utilize an interaction between the coils for preventing an electric current of a common mode noise from passing through. To be more specific, when currents in differential mode flow through the two coils, the currents flow in directions opposite to each other, so that magnetic fluxes generated by the currents cancel each other smooth the currents. However, the currents of the common mode noise flows in the same direction cause the magnetic fluxes generated in the coils to be combined together and strengthened by each other. As a result, a greater braking effect is produced due to electromotive force of the self-inductance, and prevents the current of the common mode noise from passing through.
Patent Literature 1 discloses a common mode noise filter including plural conductive coil patterns and insulating layers stacked between a pair of layers made of magnetic oxide. The pair of layers is made of Ni—Zn—Cu based ferrite, and the insulating layers are made of Cu—Zn based ferrite or Zn based ferrite.
This common mode noise filter is expected to exercise its function more effectively by getting the two coils closer to each other, thereby combining and strengthening magnetic fluxes generated. The stronger braking effect can be thus obtained. However, a closer placement of the two coils to each other will generate a large amount of a stray capacitance between the coils to produce a resonance, and prevents an electric current of a high-frequency signal from passing through.
Since electronic devices work at a higher frequency in recent years, glass-based materials are widely used for an insulating layer. In general, a dielectric constant of glass-based material which contains silica-based filler of a low dielectric constant and is used as an additive ranges from 4 to 6 while a dielectric constant of ferrite material ranges from 10 to 15. The noise filter disclosed in Patent Literature 2 includes insulating layers made of glass-based material to reduce a stray capacitance between the coils. As a result, this noise filter has better performance than a noise filter that employs insulating layers made of conventional non-magnetic ferrite material.
Patent Literature 3 discloses a ceramic electronic component and a method for manufacturing the same component. This ceramic electronic component employs a material having pores therein and a low dielectric constant. Insulating layers are laminated between a pair of coil conductors confronting each other, thereby forming a laminated body. Each of the insulating layers is made of glass-based material and has multiple pores therein. This laminated body reduces appreciably the stray capacitance between the coils. As a result, a common mode noise filter phenomenally excellent in high-frequency characteristics can be obtained.
However, in the case that the magnetic oxide layers are made of Ni—Zn—Cu based ferrite, each of the elements (i.e. magnetic oxide layers, insulating layers, and coil conductors) is made of materials different from each other. The laminated body can hardly be formed unitarily by firing these elements simultaneously free from structural failures, such as cracks or delamination between the layers. On top of that, even if an appropriate firing condition is found to the simultaneous firing of respective layers of the laminated body, and the laminated body could be formed unitarily, there is still a problem: During a heat-treat step (e.g. baking an external terminal electrode printed on the laminated body) after the firing step, cracks can be sometimes produced in the insulating layers between the coil conductors.